The present invention relates to air-conditioning systems for aircraft, and more particularly to such systems which include electrically-powered vapor-cycle cooling and use bulk fuel as a heat sink.
In the early 1960s, when it was determined that engine bleed air could be safely used in aircraft air-conditioning systems, commercial aircraft manufacturers switched from electrically-driven vapor-cycle systems to simpler, lighter weight, less expensive air cycle systems for providing pressurized, conditioned air to the aircraft cabin. Air cycle systems were considered more reliable and easier to maintain than vapor-cycle systems and the amount of fuel burn allocated to supply the high-pressure air characteristically required by the compressors and expansion turbines of the air cycle systems was not a controlling design factor.
Various improvements in the efficiency and design of early air cycle systems have been introduced. Presently, the most common type of air-conditioning system used in the commercial aviation industry is the so-called three-wheel bootstrap system. In a typical version of that system, a compressor receives regulated high-pressure and precooled bleed air from an aircraft engine(s) and delivers it via a heat exchanger to an expansion turbine. Exhaust air from the turbine, which has been cooled in the heat exchanger and further cooled by virtue of performing work in driving the turbine, is fed as conditioned air into the cabin. The expansion turbine drives the compressor (thus the term bootstrap) and also drives a fan which functions to draw ram (ambient) air through the heat exchanger.
Aircraft manufacturers have incorporated various design modifications in order to improve the efficiency of the basic three-wheel air cycle bootstrap system, such as recirculating cabin air through the system to reduce the volume of engine bleed air required. Nevertheless, the three-wheel air cycle bootstrap system remains a pressure-driven air cycle cooling system requiring engine bleed air at a pressure substantially above cabin pressure to drive a compressor and an expansion turbine. Under all flight conditions, this high-pressure bleed air can only be obtained from the compressor stages of the aircraft engines, which involves a significant fuel burn penalty. And, since high-pressure bleed air so obtained is at a commensurately high temperature, precooling is accomplished at the engine to reduce fire hazards. Additionally a substantial amount of ram air (for cooling) is required to be drawn through the bootstrap system's heat exhanger(s). These requirements result in increased drag and a further fuel burn penalty.
In the air-conditioning systems adapted for certain military aircraft, the requirement for ram air has been reduced by replacing ram air heat exchangers with heat exchangers cooled by fuel from the fuel tanks of the aircraft. Even so, these air-conditioning systems remain basically air cycle systems driven by high-pressure engine bleed air and as such, have the above-mentioned shortcomings.
Therefore, it is a general object of the present invention to provide an aircraft air-conditioning system which minimizes the requirement for engine bleed air, and thereby reduces the amount of fuel burn allocated to the air-conditioning system.
More specifically, it is a feature of the present invention to provide an aircraft air-conditioning system which taps just enough engine bleed air so as to maintain cabin pressure, and which cools the tapped bleed air with a combination of an electrically-driven vapor-cycle cooling subsystem and fuel as a heat sink.
A further feature of the invention is to provide such an aircraft air-conditioning system incorporating a vapor-cycle cooling subsystem in which the condenser for the vapor-cycle subsystem is cooled by a liquid coolant that in turn uses the fuel in the aircraft fuel tanks as a heat sink for dissipating heat accumulated by the vapor-cycle condenser.
As an additional fuel efficiency consideration, it should be noted that an air cycle system requires a fuel-burning auxiliary power unit (APU) to provide the high-pressure air needed to operate the air-conditioning system when the aircraft is on the ground and the engines are shut down. In addition to their fuel burn requirements, APUs represent added weight and present additional maintenance requirements.
Therefore, another object of the present invention is to provide an aircraft air-conditioning system which eliminates the need for an auxiliary power unit to supply high-pressure air for the cooling cycle.